Communication systems involving the use of satellites are increasingly common. Such systems can be used to transmit data, including real time communications, across large distances. In addition, satellite communication systems used in connection with devices located on or near the surface of the earth can have very large coverage areas.
In a typical satellite communication system, data is transferred between satellites, or between a satellite and communication devices located on or near the surface of the earth, using radio frequencies. Radio frequencies can be projected using broad beams, facilitating the acquisition and tracking of communication devices. However, the bandwidth of radio frequency communication systems is limited.
In order to provide higher bandwidth communications, optical systems, such as systems incorporating laser transmitters, must be used. However, free space optical communication systems have typically been limited to point to point links, where communications are between a single transmitter/receiver and another single transmitter/receiver. One reason that such optical communication systems have been limited to point to point links is that the relatively narrow beams of optical communication systems make the acquisition and tracking of communication endpoints extremely difficult.
In order to permit the acquisition and tracking of communication end points in systems where two communication end points do not have a fixed position with respect to one another, sequential acquisition and communication modes have been proposed. In particular, in the acquisition mode, a relatively broad beam is transmitted. The receiving device, upon detection of the relatively broad beam, may signal the transmitter to effect precise aiming of the signal. Communication may then commence using a communication signal having a relatively narrow beam. However, such systems are not readily adapted to the simultaneous transmission and/or reception of multiple channels at a communication endpoint.
In order to permit the simultaneous reception and/or transmission of multiple channels at a communication endpoint, satellite-based systems have been proposed that employ a telescope having a field of view that encompasses a number of communication endpoints. Communication endpoints within that field of view are located, and may be communicated with using communication signals having narrow beams. In order to position a receiver or transmitter such that it intercepts a signal beam between the multiple access device and a selected communication endpoint, the receiver and/or transmitter is mechanically positioned within the field of view of the telescope. However, such an approach does not solve the problem of potentially lengthy acquisition times, and difficulties in tracking end points that are moving with respect to the multiple access device. In addition, such an approach requires the use of relatively slow and potentially fragile mechanical devices. Furthermore, where the position of communication endpoints relative to the multiple access device are such that two or more communication endpoints occupy substantially the same position within the field of view of the telescope, the ability to distinguish between signals received from such endpoints, or to direct a transmitted signal to a selected one of such devices, is lost.
In connection with communication systems that transmit data between communication endpoints across optical fibers, different channels may be assigned different frequencies. Filters may then be cascaded to demultiplex the channels having different wavelengths. However, such an approach has not been applied to satellite communication systems capable of supporting a large number of channels. One reason for this is that the passband of filters used to separate different channels from one another moves to shorter wavelengths as the angle of incidence of the light moves from normal to the filter. Where the angle of the signal relative to the filter is known in advance, such shifts in wavelength can be compensated for. However, such an approach cannot be taken where the position of a signal, and in particular the signal's angle of incidence with respect to the filter, is not known beforehand, and therefore can lie within a range of angles.